Science: A Dish Best Served Cold: The Boomerang Nebula

Ready for another write-up about another weird-ass thing in outer space?  You know you are, and so am I!  This time I’m going to tell you about the Boomerang Nebula, which has many other names:

  • The Bow-Tie Nebula (shared with another such nebula)
  • The Matt Smith Nebula (because bow-ties are cool; OK, no one else but me calls it that)
  • The Centaurus Bipolar Nebula (pop quiz: guess which constellation it’s found in?)
  • The usual boring letters and numbers such as ESO 172-7, Magakian 587, IRAS 12419-5414, Gaia DR2 6073662099660289536, and LEDA 3074547 (there are more, but you get the idea)

The Boomerang Nebula is about four light years across and 1533 parsecs (or 5,000 light years) away from Earth in the Centaurus constellation, so gold stars to everyone who correctly guessed the constellation, and to those who didn’t, please pay closer attention when I tell you how we name stars and other spatial (special?) features.  A couple of astronomers named it for the Australian throwing stick after looking at it through the foggy, earthbound trash we called telescopes back in 1980 and thought they saw a slight curve to it. Sadly for Doctor Who fans, the name kept coming back and stuck even after the 2003 Hubble Space Telescope pictures showed the nebula was pretty even on both sides and shaped more like an hourglass or a bow-tie than a boomerang.  It’s thought to be a pre- or protoplanetary nebula, meaning that it’s brand new, probably no more than 3,500 years old.

Like LL Pegasi and Eta Carinae that I’ve covered previously for you guys, this is another “planetary nebula” (which annoyingly don’t actually have anything to do with planets – again with the inaccurate names!) that was created by an old, red giant star outgassing its outer layers as it swells up and dies.  We’ve only found about three and a half thousand of these planetary nebulae in our galaxy despite it having over 200 million stars, possibly because unlike stars nebulae only stick around for some tens of thousands of years before dissipating into space, but I promise I’m not going to write about every single one of them.  Planetary nebulae have a lot of different shapes – stellar, disk, ring, irregular, helical, bipolar, quadrupolar, wibbly-wobbly (that one’s just me again), perhaps based on factors like if it was created by a single or a multiple star system, the strength of the star’s magnetic field, orbiting planets blocking the outflow, different waves of outgassed material interacting with each other, etc.  Or maybe some of these shapes are just us seeing the same one from different angles or at different times in the evolution of a nebula.  That said, most planetary nebulae come in just one of three shapes: spherical or bubble-like (only about 20%), elliptical, or, most commonly, “bipolar,” which has sort of even, two-lobed shape on an axis.  That’s what the Boomerang Nebula is, and Eta Carinae is an especially good example of this sort of shape too.  We think this shape comes about when a star about the same size as our sun (or maybe a little bigger) goes through at least two stages of shedding its outer layers into an expanding shell of gas and dust, first more gradually, and then by firing off two strong jets from its poles (like the north and south poles on Earth), which helps define the otherwise puff-ball planetary nebula’s bipolar, two-lobed shape.  A real weird-ass thing about these bipolar nebulae is how the long axes or “wings” of those near our galaxy’s center bulge all line up with the plane of our galaxy – maybe due to a strong magnetic field at work there, but we don’t really know, because space yet withholds another dark, cold secret.  At least I won’t have to tell you what planetary nebulae are ever again, I can just link to this article in the future.

But the Boomerang Nebula is itself a weird-ass planetary nebulae, what with tossing off ten to a hundred times more gas that’s zooming away far too quickly (about 366,609 miles per hour) than happens with other dying stars.  Taken together, these details paint a picture of a literal binary star system “disaster” (from Greek, dis for “bad” + aster for “star”) where one smaller star, or maybe a white dwarf, orbited in closer over time and collided with the larger dying red star.  And that super-fast-moving gas makes the Boomerang Nebula doubly weird-ass and notably unique: it is literally the coldest place we’ve ever seen in space, even colder than ‘normal’ space.  See, I told you bow ties are cool, and this one’s the coolest.

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To understand how the Boomerang Nebula got so cold, let’s go off on a patented Megara Justice Machine Digression to understand what makes something warm, how we measure that warmth, and that famous line from Star Trek II:

  1. Temperature and heat (or lack thereof) come from the kinetic motion of the atoms and molecules that something is made up of.  It takes energy to make the atoms and molecules inside something fidget, so the less they jiggle, the less energy and heat they therefore have.  Completely stopping that jostling, or “thermodynamic motion,” puts these atoms or molecules at “absolute zero” – nothing can be any colder than that, a condition that the Kelvin temperature scale was created to express.  Kelvin is good for describing very low temperatures like this, but not so much for everyday uses, as a nice sunny day at 70 Fahrenheit and 21 degrees Celsius is 294 Kelvin, but 80 F or 27 C is still only 300 Kelvin.
  2. Khan was right when he told Kirk that “it is very cold… in space.”  Your typical outer space runs about 2.7 Kelvins, or -270.45 Celsius and -454.81 Fahrenheit, which means it still has some warmth to it, if not much.  Those meager 2.7 Kelvins come from some energy left over from the Big Bang that’s still echoing around the universe.  We call that the microwave background radiation (or “cosmic microwave background“) because it takes the form of microwaves, which are a form of electromagnetic energy near the long end of the spectrum.

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So, back to the Boomerang Nebula: that gas it’s shooting off into space from its interior star?  It’s even colder than the normal 2.7 Kelvins of outer space, thanks to its pressure.  Gas under pressure, like say in a tank, has more heat because its loose atoms or molecules are closer together and end up bouncing off one another and that tank’s walls more often, and therefore also have a higher temperature; the more pressure, the hotter the gas.  This also works in reverse, so when the Boomerang Nebula’s gas sprays away really quickly and expands, like the gas inside your sun-tan spray or perfume bottle, it’s losing pressure and therefore also heat.  The Boomerang Nebula’s wings of gas are less than half a Kelvin and are the only thing in space we’ve ever seen that are colder than the background microwave radiation – making them the perfect space spot for revenge!

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Here’s a picture taken of the Boomerang Nebula is 2003 by the Hubble Space Telescope, a 1000 second exposure made through through a green-yellow filter.

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Those wings on either side are being lit up by the center star that’s  reflecting off the dust particles in them, outgassed by the same star.  Astronomers think the whole nebula’s more or less round, but looks like a bow-tie because of dust grains orbiting the center star that are blocking some of the star’s visible light. So if we were seeing this planetary nebula from another angle, we might see the more of the traditional “blown bubble” shape.  On the other hand, it’s very new too, so it just might not have had enough time to take the more normal shape.

This more recent photo overlays an image taken by ALMA (the Atacama Large Millimeter/submillimeter Array) in 2017 on top of another, similar image from the Hubble.

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All that red stuff is the ALMA image and shows us those jets of gas (which we think are hollow) being fired off from the star’s poles, while all that blue comes from a quite similar Hubble picture of the gas that’s not moving too quickly, can be seen in visible light, and is starting to warm back up to the mean 2.7 Kelvin of normal outer space; although we haven’t seen it anywhere else yet, this cold effect may be common in planetary nebulae but just not very long-lived.  So we should enjoy this cold dish while we can.

By the way, the ALMA radio telescope (not so much a bunch of lenses in a tube but a series of radio dishes) has told us a lot about the Boomerang Nebula (and a hell of a lot of other space-stuff, both weird-ass or not), and just tweeted about this quite recently.  It won’t tell you anymore than I have here, but maybe you’ll find them to be a more trustworthy source of planetary nebulae info than myself.

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🔭   Megara Justice Machine has never owned a Members Only jacket, nor been the executive director of the Miami Space Transit Planetarium, but he does sometimes like to tell The Avocado about astronomy trivia that he knows or is willing to at least look up.  He also hasn’t worn a bow tie since prom, which was a long time ago in a galaxy far, far away.